Hydrocarbons, such as oil, gas and/or mixtures thereof, are normally found in accumulations under pressure in porous formations in bedrock. These natural hydrocarbon reservoirs are exploited by drilling one or more boreholes down into the bedrock. When drilling of these boreholes is completed and the oil well is in production, various processing installations located on or above the seabed will be able to completely or partly process the hydrocarbons.
There are some oil wells where the natural flow of hydrocarbons to the surface is not sufficient to permit or maintain commercial production. This may be due, for example, to the hydrocarbons' viscosity and/or weight, or the fact that the pressure in the oil well is too low to counteract the hydrostatic pressure of the fluid in the well together with the counter-pressure exerted on the fluid in the oil well by the processing installations.
For such oil wells a number of systems and different principles have therefore been developed which can increase the oil wells' production by means of so-called artificial lifting. The two most commonly used systems today are water injection and gas injection. With gas injection natural gas under high pressure is injected into the annular space between the casing and the tubing. For this purpose a pressure-controlled valve, a so-called gas lift valve, is usually employed in order to be able to supply and control or check the amount of gas flowing into the actual tubing.
Pressure-controlled valves of this kind can also be used during a well start-up phase, where completion fluid can be found in both the well's annulus and in the tubing. In order to start production in such a well, completion fluid must be displaced from the annulus, through one or more pressure-controlled valves, and up to the surface through the tubing.
Another similar area of application will be after a well shut-in, where fluid has filled up at least parts of the annulus or where production fluid has lain for some time and gas has migrated to the surface and where the pressure in the well is too low for the well to begin producing without receiving pressure support from gas injection.
How these pressure-controlled valves are configured and located in the well will depend on a number of parameters. For example, according to the tubing's size (diameter) and the injection pressure available, so-called gas injection points will be provided at one or more points in the tubing, whereby the specific configuration arranges for an optimal gas injection. The pressure-controlled valves, such as a gas lift valve, will then be installed in these gas injection points at the same or different depths in the tubing's longitudinal direction, with the object of initiating gas injection, thereby achieving an artificial lifting in the well.
The gas lift valve(s) can then be controlled or checked according to several different principles, for example by a pressure, where it is the pressure differential round and/or across the valve which permits a controlled opening or closing of the valve.